Historically, what was the speculum metal used by Sir Isaac Newton in the reflecting telescope?

'Speculum' metal is an alloy of copper and tin. It is a white brittle alloy of a mixture of around two-thirds copper and one-third tin. It was used historically to make different kinds of mirrors including optical devices until it was replaced by more modern materials such as metal-coated glass mirrors. It was the only practical choice in high-precision optical equipment between mid-17th and mid-19th century, before the invention of glass silvering. 'Speculum' metal was noted for its use in the metal mirrors of reflecting telescopes, and famous examples were Newton's telescope, the Leviation of Parsonstown (largest telescope from 1845 until 1917), and William Herschel's telescope used to discover the planet Uranus.

Isaac Newton was the first to successfully build a reflecting telescope in 1668. His telescope had 33-mm (1.3-inch) diameter 'speculum' metal. The composition of 'speculum' metal was further refined and ideal composition was around 68.21% copper to 31.7% tin. 'Speculum' metal was very hard to cast and shape. It only reflected 66 per cent of the light that hit it.